linux:linux.mk: Add "firmware-imx" dependency if needed

Since SDMA firmwares for imx[6,7,8] are now provided only by
firmware-imx package and not linux-firmware package [1]. If
CONFIG_EXTRA_FIRMWARE is set in the kernel config, the build will fail
if the imx firmware is not available.

[1] http://lists.busybox.net/pipermail/buildroot/2021-January/603807.html

Signed-off-by: Leger Charlie <c.leger@borea-dental.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit 666084f4943773fd659393e16982ab2baaa99d0e)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

linux/linux.mk

@@ -77,6 +77,7 @@ LINUX_DEPENDENCIES = host-kmod
 LINUX_DEPENDENCIES += \
 	$(if $(BR2_PACKAGE_INTEL_MICROCODE),intel-microcode) \
 	$(if $(BR2_PACKAGE_LINUX_FIRMWARE),linux-firmware) \
+	$(if $(BR2_PACKAGE_FREESCALE_IMX),firmware-imx) \
 	$(if $(BR2_PACKAGE_WIRELESS_REGDB),wireless-regdb)
 
 # Starting with 4.16, the generated kconfig paser code is no longer